I'd rather try shaving with a band-saw than call the flowchart reading idiots at (CenturyLink) my ISP, so I understand your pain. But, are you sure the issue is on their side of the wire?
I have seen routers on the edge start dropping packets like mad which can look like an internet issue. I have no idea how you're configured but have you made sure you can ping to and through the local hardware? And did you run the test(s) with a large unfragmented packet?
This has been an ongoing problem for the last 2 years, ever since I upgraded to get more speed. There doesn't seem to be any sort of pattern to this problem. It's not weather related, temperature related, nothing to do with the time of day. It's erratic and quite unpredictable. The only predictability to it is that it only happens on days ending with "Y".
My router/modem has been replaced twice and it has not helped. Wires, cables, splitters, and filters have all also been replaced, reversed, relocated, etc. I even set the router on top of a book like they suggested.
They finally sent a guy out here on Tuesday who tested and checked out everything inside the building from my router all the way to the line box in the basement, and he looked around outside. He says it all checks out ok and as far as he can tell, if there is a problem, it is somewhere between my building and Verizon (nothing misbehaved while he was here). He also said whoever did the original wiring in this building did a mighty fine high quality job, from layout design to workmanship. He seemed impressed. He says there doesn't seem to be any sort of leaking going on anywhere and everything was bone dry, so he doesn't think it's an issue with rain getting in and affecting the wiring.
The only thing he found that was out of sorts was an extra wire coming from my single line box in the basement that lead to "who knows where". It wasn't the one that lead straight up to my apartment and the box wasn't designed for 2 lines, so he removed it and then came back up to make sure removing it didn't negatively affect anything up here.
When he told me about it, I told him there are technically 2 separate lines coming to my apartment that each had their own number, but the 2nd one was not in service and had not been in use since the late 70's when the former owner's sister used to live in this apartment. (that was the one he removed, and it originally lead to the jack in my daughter's room). He said that wire did look kind of old and he thought that it was possible (but not probable) that the 2nd line could be causing my problems and that removing it might fix it.
Now, he was supposed to call me back on Wednesday (which he didn't), and if I was still having a connection issue, he would come back out and replace the wiring between the building and the pole. Since he didn't call me back, I am going to have to call Verizon and chances are fight with them on the phone again to either send this guy back or send another guy out.